I like that we must sometimes be careful about aggro due to bombs...keeps people from just automating button mashing. Likewise, I think being aware of ones environment is an important skill that divides good players from bad.
We can agree to disagree =)
I also dont agree with your assesment that forcing players to tag will not fix the FO role.
If you heal AoE, you will potentially heal the "spy"; you can't control who your AoE heal touches and who it doesn't, beyond controlling where you drop your bomb or aim your spray/ricochet.
As long as they appear to be trying to help by throwing out level 1 sap heals, they'll be considered a contributing member of the attack, be res'd as needed, and continue to provide important intel to comrades in voice comms or other means. I don't know many people who stop in the middle of battle to carefully look around at how much everyone is contributing and compare the output of their abilities to the impact that the hp/sap credits appear to take. Personally, I'm more interested in my target's information, and finding the next target to affect.
Neutral, or tagged friendly, the enemy is still obtaining important information about your side of the battle, particularly the info that can be gained by following you from the respawn point.
This information leak seemed to be your main concern, and tagging does not address it at all.